Former archery instructor checking in. An arrow from a 35 lb draw bow at half draw certainly isn't going to kill anyone unless you somehow manage to hit them in the eye, and doesn't fly fast enough (read: straight enough - slow arrows flutter) to even be reliably accurate past 10-20 yards anyways. This isn't a practical technique, but rather a demonstration of his method of speed reloading.

That said, what this guy is doing is very impressive. The ability to go right into your draw from an arrow in your hand instead of using a quiver would be an extremely useful technique for a hunter or an archer on the battlefield. If you take his reloading technique, take an extra half second to get to full draw, and an extra two seconds to aim, now you're putting a killing arrow down range every 3 seconds. That is really, really cool. There's a lot of haters on his video because he's doing something different, and people entrenched in traditional ways of learning tend to equate "different" and "wrong".

The chainmail protects against slashing blows. It's next to useless against stabbing attacks, arrows, and especially blunt weapons (like an axe or hammer), as the weight actually gets dispersed on impact resulting in the chainmail literally tearing into the skin, producing horrible wounds that were tremendously painful, almost gauranteed to get infected and were the majority of the time leathal as it was almost impossible for the skin to regrow on such a large and open wound.

What you would really want, to compliment your chainmail (medieval soldiers wore multiple types of armor at the same time to protect against different threats), would be thick wool padding, boiled leather (the combination of the dense, thick wool with the leather did a tremendous amount to blunt the force of many kinds of attacks including piercing arrow fire) and then ring or chainmail on top of that to guard against slashing.

This would have been much more common to see, especially on European knights in the medieval era. Plate armor was actually quite rare and expensive in comparison, and you'd likely never see anyone, even a knight or royalty in full plate armor on the battlefield. You'd have to be enormously wealthy to wear plate into battle, as the suites had to be tailer made to the customer by artisan armor smiths.

And in the east, of course, plate was seen, but lamaller and scale type armors were much more common among the wealthy and warlike classes of society like the mamlukes and the cataphracti.